Welding method

ABSTRACT

Two aluminium alloy work-pieces are welded together. Firstly, a portion ( 2 ) of each work-piece ( 1 ) is prepared, the preparation including the performance of a surface treatment, such as friction stir welding, that results in a region (A) extending from the exterior surface into the work-piece having a grain structure that is finer than the grain structure of the work-piece outside (region C) that region. Then the work-pieces are welded together by means of a fusion welding process, such as electron beam welding, that joins the respective prepared portions ( 2 ) of the two work-pieces ( 1 ). The preparation of the work-pieces is so performed that said region (A) extends into the work-piece to a depth that exceeds the depth of material (region D) that is caused to melt during the fusion welding process.

BACKGROUND OF THE INVENTION

The present invention relates to welding two metal work-pieces togetherand relates in particular, but not exclusively, to welding together tworelatively thick work-pieces made from metal alloys that have beenprepared for use in the manufacture of aircraft components.

When manufacturing aircraft components it is sometimes necessary ordesirable to weld together thick work-pieces for example solid blocks ofaluminium alloy having a thickness of 50 mm or greater. The alloy willtypically be an aluminium alloy that has been treated so that it hascertain mechanical properties necessary for the alloy to be suitable foruse in the manufacture of aircraft components. As a result, themicrostructure of the alloy is formed of relatively coarse elongategrains that are generally oriented parallel to each other. Weldingblocks of aluminium of such a thickness is generally performed by meansof a fusion welding process such as an electron beam (EB) weldingprocess. It is common when joining blocks of aluminium in this way forcracks to form (for example micro-cracks) in or near the region of theweld. Such cracks weaken the welded component particularly under fatigueloading.

SUMMARY OF THE INVENTION

It is an object of the present invention to produce a method of weldingtwo work-pieces together that eliminates or reduces the formation ofcracks during the welding process as compared to the known prior artmethod described above.

According to the present invention there is provided a method of weldingtogether two metal work-pieces, the method including the followingsteps:

-   -   providing two metal work-pieces,    -   preparing a portion of each work-piece, the preparation        including the performance of a surface treatment that results in        a region extending from the exterior surface into the work-piece        having a grain structure that is finer than the grain structure        of the work-piece outside that region, and    -   welding together the work-pieces by means of a fusion welding        process that joins the respective prepared portions of the two        work-pieces,    -   wherein said region extends into the work-piece to a depth that        exceeds the depth of material that is caused to melt during the        fusion welding process.

It has been found that the treating of the metal in the region of theweld joint to be formed mitigates the problems associated with theformation of cracks. It has been observed that the formation of at leastsome cracks in the methods of the prior art tends to occur in thework-piece near the interface between the alloy and the weld, in regionswhere there has been partial liquefaction of the alloy due to the heatgenerated when welding. Such cracks are often referred to as liquationcracks and commonly occur at the boundary between adjacent grains in thealloy. It is thought that such cracks result from the formation of grainboundary liquid (at temperatures below the melting point of the grainsof the alloy), which, being unable to support the tensile stresses thatare developed as a result of the temperature changes during and afterwelding, leads to cracks forming along the grain boundary.

The average maximum dimension of the grains in the work-piece outsidethe region that has been subjected to said surface treatment ispreferably at least five times greater than the average maximumdimension of the grains in the work-piece inside the region.Alternatively or additionally, the method is performed such that thereis at least one cross-section in which the difference between the grainsize number of the material in the work-piece outside the region thathas been subjected to said surface treatment and the grain size numberof the material in the work-piece inside the region is greater than orequal to 4. The grain size number, G, of a material is defined by theequation n=2^(G−1), where n=the number of grains per square inch at 100×magnification (i.e. the number of grains in an area of 0.0645 mm²).

The preparation of the work-pieces may, if necessary, include a step oftreating or machining the surface treated regions of each work-piece toproduce a surface on one work-piece that can be fusion welded to acorresponding surface on the other work-piece. The regions subjected tosaid surface treatment may for example be skimmed to producesubstantially flat surfaces. The skimming of the work-pieces may forexample be performed by a milling machine. The skimming may typicallyremove a layer of material from a face of the work-piece that, had thework-piece not been subjected to said surface treatment, would have beenabout 0.5 mm thick. The depth of material removed during skimming willof course depend on the amount of material that needs to be removed inorder to provide a flat surface, which will of course depend on thesurface treatment employed.

The surface treatment of the metal is preferably performed within aregion that encompasses the region that will liquefy during the fusionwelding process. The region that will liquefy may for example at thelowest be about 2 mm, and may be as high as 5 mm (or even higherdepending on the fusion welding method employed), to either side of theweld joint. In the case where the depth of liquefaction during fusionwelding is 2 mm to either side of the joint, the surface treating of themetal beforehand must extend beyond that depth, for example to a depthof at least 5 mm. In the case where the surface of the work-piece isskimmed between the steps of surface treatment and fusion welding, thenthe depth of the material that has been subjected to said surfacetreatment will of course be reduced. The surface treatment may beconducted to a depth of at least 10 mm. The surface treatment may beconducted to a depth of less than 100 mm. The surface treatment may forexample be conducted to a depth of between 5 mm and 40 mm and morepreferably to a depth of between 10 mm and 30 mm. It will of course beunderstood that the surface treatment is referred to as such onlybecause the treatment is effected near the surface of the work-piece andthat the term is not limited to treatments where the treatment affectsonly the surface properties of the work-piece.

It will be appreciated that, depending on the depth to which the metalis subjected to said surface treatment, the fusion welding process maygenerate a heat affected zone (i.e. a zone in which the mechanicalproperties of the metal/alloy, such as for example hardness, are alteredby the heat generated during welding) that extends beyond the boundaryof the metal that has been subjected to said surface treatment.

The surface treatment is preferably so performed that the temperature ofthe metal does not reach the melting temperature of the metalwork-piece. Advantageously, the surface treatment is performed such thatthe temperature of the metal does not exceed the eutectic phase meltingtemperature. Preferably, the surface treatment is performed such thatthe temperature of the metal does not exceed the liquation temperatureof the metal in the grain boundaries.

Advantageously, the performance of the surface treatment causesplasticization of the metal, but preferably causes substantially noliquefaction or fluidization. Preferably, the surface treatment causessubstantially no melting of the metal.

Advantageously, the surface treatment is performed by means of afriction stir welding process. Such a process is described in U.S. Pat.No. 5,460,317 (Thomas et al), U.S. Pat. No. 5,813,592 (Midling et al),WO 93/10935 (The Welding Institute), and WO 95/26254 (Norsk Hydro A.S.), the specifications of which are hereby fully incorporated herein byreference thereto. The friction stir welding process may be in the formof a process as described in any of those references. It will beunderstood that the term “friction stir welding” encompasses any methodof welding in which a probe of material harder than the work-piecematerial is caused to move relative to the work-piece to generatefrictional heat causing the work-piece in the region of the probe tobecome plasticised, the probe effectively entering the work-piece. Theprobe is conventionally caused to rotate about the probe axis and tomove along the work-piece along the length of the weld to be formed.

The fusion welding process is conveniently performed by means of anelectron beam welding process. Other fusion welding processes could beutilised but, of the methods currently available, electron beam weldingis preferred because of the depth of weld achievable at relatively lowweld widths.

The work-pieces may be made from low-density alloys. For example, thedensity of the metal is preferably less than 5,000 Kgm⁻³, morepreferably less than 4,000 Kgm⁻³, and yet more preferably less than3,000 Kgm⁻³. The work-pieces are preferably made from lightweightalloys. The work-pieces may be made from aluminium alloys. Thework-pieces may be made from cold-worked metal. The cold-worked metalmay for example have been subjected to a rolling process. The method isof particular application in the case where the work-pieces are suitablefor use in the manufacture of an aircraft or aerospace component. Forexample, the metal may be any conventional or suitable alloy used in theaerospace industry, such as 2000 series, 6000 series, 7000 seriesaluminium alloys, or aluminium-lithium alloys. Such alloys have in thepast been viewed as being difficult to weld together satisfactorily,especially where the depth of the joint to be welded is greater than orequal to about 50 mm. Such alloys may be difficult to weldsatisfactorily due to one or more of several factors including a) thecomplexity of the alloying system, b) the particular heat treatment(s)to which the alloy has previously been subjected, c) themechanical/chemical structure/composition of the material, d) theparticular arrangement of different phases in the alloy or ofprecipitates formed in the alloy and/or e) the size and/or orientationof the grains of the alloy material.

The two work-pieces may, but need not be, of the same type of material.The method of the present invention is for example advantageously ableto be used to weld together different metals or alloys. For example, themethod of the present invention could be utilised when manufacturing acomponent, one part of which being required to have one set ofmechanical/physical properties and another part of the component beingrequired to have a different set of mechanical/physical properties.

The method may of course be performed to weld together one or more otherwork-pieces, possibly welding the multiplicity of work-pieces togethersimultaneously or possibly welding the multiplicity of work-piecestogether in series (sequentially).

The work-pieces may be in the form of blocks of material. The blocks ofmaterial after having been welded together may for example be machinedinto a component. The present invention thus also provides a method ofmanufacturing a component, for example an aircraft component, whereinthe component is machined from a block of metal, the block of metalhaving been made from two or more work-pieces welded together inaccordance with the method according to the present invention asdescribed above. The block or blocks may conveniently, but notnecessarily, be cuboid in shape. The size and shape of the work-piecesto be welded together may be, and possibly need only be, limited by thelimitations of the fusion welding process employed. For example, it ispossible to weld with a 120 kW (120 kV operating at up to 1000 mA)electron beam welding apparatus to depths of up to 450 mm. With such anapparatus it would be possible to weld together two work-pieces having athickness of 450 mm by means of a single pass electron beam weld. If adual pass (i.e. one welding pass on each side of the joint to be welded)electron beam weld process is utilised, the thickness of work-piecesable to be joined could be as high as 900 mm. Greater thicknesses ofmaterial could be welded together with more powerful fusion weldingequipment. Whilst a dual pass electron beam welding process is possible,a single pass process is preferred, because of the potentialdifficulties in ensuring a high quality weld joint in the region wherethe two electron beam welds interface.

It will be appreciated that the work-pieces may be trimmed and/ormachined after performance of the method of the present invention andthat therefore the integrity of the weld joint in the regions of thework-piece that are subsequently removed is not important. There mayalso be other regions where the integrity of the joint between thework-pieces is not important for other reasons. For example, thework-pieces once joined might be machined into a component that in useis subjected to forces/stresses such that the strength of weld jointrequired varies across the joint. In such cases, the portions of therespective work-pieces being welded together in accordance with thepresent invention may actually be contained within larger regions thathave been subjected to a surface treatment, or similarly prepared.

According to another aspect of the invention there is provided a methodof manufacturing an aircraft component including the following steps:

-   -   providing two metal work-pieces,    -   preparing a portion of each work-piece, the preparation        including the performance of a surface treatment that results in        a region extending from the exterior surface into the work-piece        having a grain structure that is finer than the grain structure        of the work-piece outside that region, and    -   welding together the work-pieces by means of a fusion welding        process that joins the respective prepared portions of the two        work-pieces,    -   wherein said region extends into the work-piece to a depth that        exceeds the depth of material that is caused to melt during the        fusion welding process.

According to a further aspect of the invention there is provided amethod of welding together two metal work-pieces, the work-pieces beingmade from a lightweight alloy suitable for use in the manufacture of anaircraft component, the method including the following steps:

-   -   providing two metal work-pieces,    -   preparing a portion of each work-piece, the preparation        including the performance of a surface treatment that results in        a region extending from the exterior surface into the work-piece        having a grain structure that is finer than the grain structure        of the work-piece outside that region, and    -   welding together the work-pieces by means of a fusion welding        process that joins the respective prepared portions of the two        work-pieces,    -   wherein said region extends into the work-piece to a depth that        exceeds the depth of material that is caused to melt during the        fusion welding process. The present invention also provides a        component made from two work-pieces welded together in        accordance with the method of the present invention as described        herein. The component may be in the form of an aerospace        component, an aircraft component or any other component that is        required to have similar alloy material properties. The        component may for example be in the form of a spar for an        aircraft wing box. The spar may be over 10 metres long. The        billets of the alloy that are supplied to the aircraft        manufacturer may have a maximum dimension of 5 metres. Such a        spar may be manufactured from those billets by welding a        plurality, three for example, billets end to end by means of the        method of the present invention and then machining the spar from        the resulting block.

The invention further provides a component comprising a weld jointjoining one part of the component to an adjacent part of the component,the component in the region of the joint comprising a portion, that hasbeen fusion welded, sandwiched between two portions that have each beenfriction stir welded. The component may be in the form of an aerospacecomponent, an aircraft component or any other similar component.

It will be appreciated that features described with reference to oneaspect of the invention may be incorporated into other aspects of theinvention. For example, the component, for example an aircraftcomponent, according to the present invention may be made by means ofthe method of the present invention.

It will be appreciated that a component formed by means of the presentinvention may require further processing before assembly. The componentmay therefore, in certain circumstances, be considered as anintermediate, requiring further processing before being considered as afinished article. For example, the component may require furthermachining, treating, assembly with other parts, or any other suchprocesses. It will therefore be understood that the term component isused herein both to cover the case where the component is in a stateready for final assembly and the case where the component is at anearlier stage in the component's manufacture.

According to a preferred aspect of the invention there is also provideda method of welding together two work-pieces, the method including thefollowing steps:

-   -   providing two metal work-pieces,    -   friction stir welding a region of each work-piece,    -   preparing the friction stir welded regions of each work-piece to        produce a surface on one work-piece that can be fusion welded to        a corresponding surface on the other work-piece, and    -   fusion welding the respective prepared surfaces of the two        work-pieces together, thereby joining the work-pieces.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, of which:

FIG. 1 a illustrates schematically the welding together of two metalplates according to a first embodiment of the invention;

FIG. 1 b shows a portion of one of the plates shown in FIG. 1 a during afriction stir welding process;

-   -   FIG. 1 c shows a cross-section of the portion of the plate shown        in FIG. 1 b;

FIG. 2 a shows a cross-section of the final welded joint formed by meansof the first embodiment;

FIGS. 2 b to 2 f show magnified portions of FIG. 2 a;

FIG. 2 g shows a portion of FIG. 2 a illustrating the different regionsof the weld;

FIG. 3 shows a magnified portion of an alloy that has been friction stirwelded;

FIG. 4 shows a magnified portion of an alloy that has been electron beamwelded in accordance with a prior art method;

FIG. 5 illustrates schematically the welding together of three aluminiumbillets according to a second embodiment of the invention; and

FIG. 6 shows a wing spar machined from two billets in accordance with athird embodiment.

DETAILED DESCRIPTION

The first embodiment of the invention relates to an experiment in whichtwo rolled plates were welded together. The rolled plates 1 before beingjoined are illustrated schematically in FIG. 1 a. The plates were madeby rolling in the longitudinal direction of the plates (represented byarrow L in the drawings). The direction of the width of the plate isrepresented by arrow LT (i.e. the Long Transverse direction). Thedirection of the thickness of the plate is represented by arrow ST (i.e.the Short Transverse direction). The rolled plates 1 have a thickness t(in direction ST) of 150 mm. The plate was made from a 7000 seriesaluminium alloy in T7651 temper condition. The alloy used comprisesAluminium, Zinc, Copper and Magnesium. The alloy had a relative highcontent of zinc (>6 wt. %). This alloy was chosen because of the knowndifficulties associated with fusion welding the alloy.

The microstructure of the respective sides 2 of the plates 1 to bejoined was modified using a friction stir welding process. The tool usedin friction stir welding process had a 30 mm shoulder diameter and a pinhaving a length of 12.05 mm. The pin used has a cross-section thattapers (the cross-section becomes progressively smaller) along itslength, the angle of the taper being 10 degrees, from a diameter of 14mm at its widest at the top of the pin (immediately beneath theshoulder). Ten overlapping bead-on-plate weld runs were performed in theLT (Long Transverse) direction on the side 2 of each plate 1 using thefollowing welding parameters: tool rotation=190 rpm, welding speed=150mm/min and vertical force (i.e. down the length of the tool)=61 kN. Toavoid material overheating during the welding process, on completion ofeach weld run (each bead-on-plate weld) the plate was allowed to cool toroom temperature before the next weld run was commenced. FIG. 1 b showsschematically a portion of the plate 1 including the side 2 of the plateafter welding. FIG. 1 c shows the plate in cross-section (the sectionbeing taken across plane C-C, which plane has a normal axis that isparallel to the LT direction). As can be seen in FIG. 1 c, thesuccessive weld runs 3 were performed such that there was a 10 mmseparation ws between neighbouring weld centre-lines w₁, w₂, w₃ . . .w₁₀. The tool achieved a 12 mm weld penetration. Thus, as can be seen inFIG. 1 b, a welded region having a width ww of at least 100 mm (in theST-direction) and a depth wd of about 12 mm deep (in the L-direction)was formed. As such, the parent material structure (having a coarsegrain structure) was changed into a typical friction stir weld structure(a fine grain structure).

After the friction stir welding step was completed, the top surfaces ofthe welds were skimmed, thereby removing about 1.0-1.5 mm of materialfrom the sides 2 to be welded together, thereby forming a smooth flatsurface. Both plates were also machined to trim their thickness (in theST direction) so that the surface of the side 2 of the plate 1 to bewelded was 100 mm thick, the entire surface on that side 2 thus havingbeen affected by the friction stir welding process (thereby providing afine grain structure).

The two plates 1 were assembled in a vacuum chamber with the use of tackwelds to form a 100 mm thick (in the ST direction) butt joint running inthe LT direction. Run-in and run-out plates were positioned either sideof the butt joint and the joint was backed by a backing plate. Electronbeam welding was then performed horizontally in the LT direction with avertical beam and using the following welding parameters: acceleratingvoltage=60 kV, beam current=450 mA, focus current=610 mA, weldingspeed=240 mm/min, vacuum in the chamber=2×10⁻⁴ torr, beamoscillation=1.2 mm diameter circle and an oscillation frequency=800 Hz.

The welded joint so produced is shown in FIGS. 2 a to 2 g. FIG. 2 ashows a cross-section of the weld joint, the section being taken in theplane parallel to the ST and L directions and having its normal axisparallel to the LT direction. FIG. 2 g shows a portion of FIG. 2 a(rotated by 90 degrees) illustrating the various regions A, B, C, D ofthe weld. As can be seen in FIGS. 2 a and 2 g, the electron beam weld(region D) is formed between the two plates 1 and is sandwiched betweenthe friction stir welding regions A on each respective plate 1. Beyondthe friction stir welding region A is the parent alloy of the plate 1,represented by region C. The interface between regions A and C isrepresented by region B. Region B, being relatively narrow compared toregions A and C, is represented in FIG. 2 g by the dotted white linethat divides regions A and C.

The average width of the electron beam weld is about 5 mm. The averagewidth of regions A-D-A combined is about 20 mm. The width of the heataffected zone of the electron beam weld is very approximately 30 mm.

FIG. 2 b shows a region of FIG. 2 a magnified to show the grainstructure at the interface (region B) between the friction stir weldregion A and the parent alloy (region C). The left-hand side area ofFIG. 2 b shows the shearing of the alloy and shows that the grainsbecome progressively smaller as one moves from region C (the right ofFIG. 2 b) to region A (the left of FIG. 2 b). FIG. 3 shows a separatesample in cross-section illustrating more clearly the size andorientation of the grains in regions A, B, and C. As can more clearly beseen in FIG. 3, the grains in region A (the region that has beenfriction stir welded) are much finer than the coarse grains in region C(of the unwelded parent alloy). It will be observed that no cracks areapparent in either FIG. 2 b or 3. The difference between the grain sizenumber, G_(A), of the alloy in region A and the grain size number,G_(C), in region C is greater than 3.

FIG. 2 c shows a region of FIG. 2 a magnified (at the same magnificationas FIG. 2 b) to show the grain structure at the interface (region B)between two neighbouring friction stir welds and the parent alloy(region C). Again, whilst the grains have been sheared, the transitionbetween the parent alloy (to the right in FIG. 2 c) and the adjacentfriction stir welded regions (to the left in FIG. 2 c) is gradual. Itwill again be observed that no cracks are present.

FIG. 2 d shows a region of FIG. 2 a magnified (at about 2.5 times themagnification of FIGS. 2 b and 2 c) to show the grain structure in thefriction stir welded region A. The grains in region A are relativelyfine compared to the grains in region C (taking into account thedifference in magnification between FIGS. 2 b and 2 c on the one handand FIG. 2 d on the other). Again, there is no evidence of any crackingor faults.

FIG. 2 e shows a region of FIG. 2 a magnified (at the same magnificationof FIG. 2 d) to show the grain structure at the interface between thefriction stir weld region A (the right hand side of FIG. 2 e) and theelectron beam welded region D (the left hand side of FIG. 2 e). Thegrains in this interface region are relatively fine. The interfacebetween the two regions is gradual and therefore difficult to identify,especially as the grain size and orientation in each region are verysimilar. FIG. 2 e shows however that the interface between the electronbeam weld and the friction stir weld region is of very high quality. Yetagain, there is no evidence of any cracking or faults.

FIG. 2 f shows a region of FIG. 2 a magnified (at the same magnificationof FIGS. 2 d and 2 e) to show the grain structure within the electronbeam welded region D. Again, the grains in this interface region arerelatively fine and are of a similar size to, although very slightlylarger than, the grains in the friction stir welded region A. Whilstnone would be expected in any case, it will be seen that no cracks areevident in this region D.

FIGS. 2 a to 2 g illustrate that the present embodiment may be utilisedto produce high quality welds, without liquation cracking, on alloyswhere it has generally been considered difficult, if not impossible, toform welds on joints having any substantial thickness. The limit on thethickness of the joint of the present invention will probably bedetermined by the limit of the thickness to which the fusion welding (inthis embodiment, electron beam welding) can be effected satisfactorily.

By way of comparison, FIG. 4 shows a cross-section of a joint madebetween two plates of the same alloy as used in the first embodiment,without the step of friction stir welding. The electron beam weld isshown as region D and is sandwiched directly between two regions C ofparent alloy (of the two plates, respectively). As can be seen in FIG.4, cracks E have formed as a result of the electron beam welding.

FIG. 5 illustrates schematically a second embodiment of the presentinvention. Three billets 1 of aluminium alloy suitable for forming anaircraft component are welded together end to end to form an elongateblock of aluminium alloy. Each billet measures 5 m×2 m×200 mm. Adjacentend faces 2 of the billets 1 are welded together by means of a methodsimilar to that described above in relation to the first embodiment ofthe invention. Almost the entire surface of each end face 2, of eachbillet to be welded to an adjacent billet, is friction stir welded to adepth of 25 mm. Then the end faces so welded are skimmed by means of amilling machine that removes about 1 mm of material from the end face.The top and bottom faces, abutting the end face, are also skimmed inpreparation for the next step. Adjacent billets are then welded togetherby means of an electron beam welding process, thereby forming a solidblock of alloy measuring about 15 m×2 m×200 m. A spar for an aircraftwing is then machined from the single solid block. The spar is about 14m long.

According to a third embodiment, shown in FIG. 6, two billets ofdifferent alloy material are welded together by means of the method ofthe second embodiment described above, although only two billets arejoined in this embodiment. A first billet of 2000 series alloy measuring100 mm×1 m×10 m is joined to a second billet of 7000 series alloy alsomeasuring 100 mm×1 m×10 m, thereby forming a block of material measuring100 mm×2 m×10 m. The resulting block is then machined into a spar, suchas that shown in FIG. 6. The spar has an upper portion 4 made of 7000series alloy providing a high strength region, where strength isimportant, and a lower portion 5 made of 2000 series alloy providing aregion where a high damage tolerance is more important than strength.The weld line between the two portions 4, 5 is labeled with referencenumeral 6 in FIG. 6

It will, of course, be appreciated that various modifications may bemade to the above-described embodiments without departing from thespirit of the present invention. For example, components (such as forexample a wing rib or a section of the wing skin) other than a wing sparcould be machined from the billets of aluminium once welded together.Rather than electron beam welding, other fusion welding processes couldbe employed, such as laser welding. The invention has application inrelation to alloys other than Aluminium alloys including for exampleMagnesium alloys. Other modifications will, of course, be apparent tothe person skilled in the art.

1. A method of welding together two metal work-pieces, the methodincluding the following steps: providing two metal work-pieces,preparing a portion of each work-piece, the preparation including theperformance of a surface treatment, including a friction stir weldingprocess, that results in a region extending from the exterior surfaceinto the work-piece having a grain structure that is finer than thegrain structure of the work-piece outside that region, and then weldingtogether the work-pieces by means of a fusion welding process that joinsthe respective prepared portions of the two work-pieces, wherein saidregion extends into the work-piece to a depth that exceeds the depth ofmaterial that is caused to melt during the fusion welding process.
 2. Amethod according to claim 1, wherein the surface treatment is conductedto a depth of at least 10 mm into each work-piece and wherein the depthof the joint to be fusion welded is greater than 50 mm.
 3. A methodaccording to claim 1, wherein the surface treatment is performed suchthat there is at least one cross-section in which the difference betweenthe grain size number of the material in the work-piece outside theregion that has been subjected to said surface treatment and the grainsize number of the material in the work-piece inside the region isgreater than or equal to
 4. 4. A method according to claim 1, whereinthe step of preparing the work-pieces includes a step of treating ormachining the surface-treated regions of each work-piece to produce asurface on one work-piece that can be fusion welded to a correspondingsurface on the other work-piece.
 5. A method according to claim 1,wherein the fusion welding process is performed by means of an electronbeam welding process.
 6. A method according to claim 1, wherein thework-pieces are made from aluminium alloys.
 7. A method according toclaim 1, wherein the work-pieces are made from cold-worked metal.
 8. Amethod according to claim 1, wherein the work-pieces are suitable foruse in the manufacture of an aircraft component.
 9. A method of weldingtogether two work-pieces, the method including the following steps:providing two metal work-pieces, friction stir welding a region of eachwork-piece, preparing the friction stir welded regions of eachwork-piece to produce a surface on one work-piece that can be fusionwelded to a corresponding surface on the other work-piece, and fusionwelding the respective prepared surfaces of the two work-piecestogether, thereby joining the work-pieces.
 10. A method of weldingtogether two metal work-pieces, the method including the followingsteps: providing two metal work-pieces, preparing a portion of eachwork-piece, the preparation including the performance of a surfacetreatment that results in a region extending from the exterior surfaceinto the work-piece having a grain structure that is finer than thegrain structure of the work-piece outside that region, and weldingtogether the work-pieces by means of a fusion welding process that joinsthe respective prepared portions of the two work-pieces, wherein saidregion extends into the work-piece to a depth that exceeds the depth ofmaterial that is caused to melt during the fusion welding process.
 11. Amethod according to claim 1, wherein the work-pieces, when weldedtogether, form at least part of a block of metal, the method furtherincluding the step of manufacturing an aircraft component, wherein theaircraft component is machined from the block of metal.
 12. A methodaccording to claim 9, wherein the work-pieces, when joined, form atleast part of a block of metal, the method further including the step ofmanufacturing an aircraft component, wherein the aircraft component ismachined from the block of metal.
 13. A method according to claim 10,wherein the work-pieces, when welded together, form at least part of ablock of metal, the method further including the step of manufacturingan aircraft component, wherein the aircraft component is machined fromthe block of metal.
 14. A method according to claim 1, wherein themethod further includes a step of making an aircraft component from thework-pieces when welded together, and a step of manufacturing anaircraft including the aircraft component.
 15. A method according toclaim 9, wherein the method further includes a step of making anaircraft component from the work-pieces when welded together, and a stepof manufacturing an aircraft including the aircraft component.
 16. Amethod according to claim 10, wherein the method further includes a stepof making an aircraft component from the work-pieces when weldedtogether, and a step of manufacturing an aircraft including the aircraftcomponent.
 17. A component comprising a weld joint joining one part ofthe component to an adjacent part of the component, the component in theregion of the joint comprising a portion, that has been fusion welded,sandwiched between two portions that have each been friction stir weldedprior to the formation of the fusion welded portions.
 18. A componentaccording to claim 17, wherein the component is an aircraft component.19. A component made from two work-pieces welded together in accordancewith the method as claimed in claim
 1. 20. A component made from twowork-pieces welded together in accordance with the method as claimed inclaim
 9. 21. A component made from two work-pieces welded together inaccordance with the method as claimed in claim 10.